Studies are being continued concerning the mechanism of plasma membrane passage of purines, pyrimidines and their nucleosides. Similar study is in progress concerning inhibitors of dihydrofolate reductase of diaminopyrimidine type. The amino acid sequence of dihydrofolate reductase purified from MTX-resistant sarcoma 180 cells is being determined. The enzymatic and cellular basis which is responsible for the greater sensitivity of mouse cells, as compared to human cells, to 5-fluoropyrimidines is being investigated. BIBLIOGRAPHIC REFERENCES: Grindley, G.B., Lowe, J.K., Divekar, A.Y. and Hakala, M.T. Potentiation by Guanine Nucleosides of the Growth Inhibitory Effects of Adenosine Analogs on L-1210 and Sarcoma 180 Cells in Culture. Cancer Res., 36, 379-383, 1976. Hakala, M.T. Uptake of Drugs and Precursors. UICC Technical Report Series, Vol. 21, 2-7, 1976.